CIDNP

CIDNP (Chemically Induced Dynamic Nuclear Polarization), often pronounced like "kidnap", is a non-Boltzmann nuclear spin state distribution produced in thermal or photochemical reactions, usually from colligation and diffusion, or disproportionation of radical pairs, and detected by NMR spectroscopy as enhanced absorption or emission signals. CIDNP was discovered in 1967 by Bargon and Fischer, and, independently, by Ward and Lawler. Early theories were based on dynamic nuclear polarisation (hence the name). The subsequent experiments, however, have found that in many cases DNP fails to explain CIDNP polarization phase. In 1969 an alternative explanation was proposed by Closs, and, independently, by Kaptein and Oosterhoff, which relied on the ability of nuclear spin interactions to alter the recombination probability in reactions that proceed through radical pairs. This mechanism, known as the Radical Pair Mechanism is currently accepted as the most common cause of CIDNP. There are, however, exceptions, and the DNP mechanism was found to be operational, for example, in many fluorine-containing radicals.